In 2009 Dell was advertising a super high end laptop that supported dual monitors at 2560 * 1600. At that time, I think the storage was two 500G drives. Monitor was 17" (May have also been available in 15", can't remember). I was lusting heavily! I think the price was in the 4K - 5K range .
I tried to pull this up for the thread, but having difficulty finding it, I may need to call Dell. In any case it IS quite rare to find this ability. I had never seen it prior to this Dell offering in a laptop. Most of the high-end cards for desktop already have this (ie. such as the Nvidia GTX 260 and ATI equivalent and up). If that price is in your range, then I suggest calling Dell to see if the sales tech can point you to the model. It may have been too expensive to catch on, but I am absolutely sure it was there. I will post update when I locate this model and see if it still obtainable.